Flame glossing rod-shaped single crystals of corundum



g- 26, 1952 M. H. BARNES ET AL 2,608,031

FLAME GLOSSING ROD-SHAPED SINGLE CRYSTALS OF CORUNDUM Filed April 16, 1946 2 SHEETS-SHEET l \NVENTORS MALCOLM H.B.4RNES BYEDWARDL .MCCANDLESS ATTORNEY Aug. 26, 1 M. H. BARNES ET AL 2,608,031

FLAME GLOSSING ROD-SHAPED SINGLE CRYSTALS OF CORUNDUM File d April 16, 1946 2 suEET s sHEET 2 1%. 6 a. SAPPHIRE V SPINEL POLISHED WITH DIAMOND POWDER POLISHED WITH Al 0 POWDER SAPPHIRE SPINEL FLAME GLOSSED FLA ME GLOSSED INVENTORS ATTO R N EY MALCOLM H. BARNES EDWARD L.MCCANDLESS Patented Aug. 26, 1952 FLAME GLOSSING ROD-SHAPED SINGLE CRYSTALS OF COBUNDUM Malcolm H. Barnes and Edward L. McCandless,

-Kenmore, N. Y., assignors, by mesne assignments, to Union Carbide and Carbon Corporation, a corporation of New York Application April 16, 1946, Serial No. 662,454

2 Claims.

This invention is a process for providing glossy scratch-free surfaces on crystalline bodies of synthetic corundum. The invention also relates to crystalline corundum products having glossy scratch-free surfaces. This application is a con tinuation-in-part of our application 504,544 filed October 1, 1943, now abandoned.

Both natural and synthetic corundum crystals have been available for years in the form of clear sapphire (pure alumina) and ruby (alumina with a small percentage of chromium oxide). Such stones have rough surfaces when found in nature or when synthesized. For example, boules and thin rods of synthetic corundum, in the as-grown condition, have matte or frosted surfaces composed of many microscopic angular crystalprojections or facets. Also, when such stones. are sawed or ground, their surfaces are covered with scratches and projections pro- .duced'by particles of abrasive material.

Before a hard body of corundum may be used as a jewel bearing, thread guide,'or other lowfriction wear-resistant device, or as a gem stone, it must be given a smooth and glossysurface, which has been done for generations by rubbing with successively smaller particles of diamond powder or like abrasive. While such .mechanically polished surfaces may appear to be smooth and uniform to the naked eye, a microscope reveals the presence of many small scratches and pits produced by the small 'particles of abrasive. Furthermore, mechanical polishing-is a slow, tedious, and expensive process. 7

The principal object of the presentinvention is the provision of a novel process for rapidly and inexpensively treating crystalline corundum bodies so as to give them glossy surfaces free from pits and scratches. Other objects are the provision of such a process whereby glossing is accomplished solely by heat; whereby the original shape of the body is retained; and whereby the character of the surface finish may be accurately controlled. Stillanother object is to impart utility to synthetic corundum rods of inferior quality. Other objects are the provision of: crystalline corundum bodies having glossy surfaces free from'notches, pits and scratches; thin rods of synthetic corundum having substantially even and glossy surfaces free from pits and scratches; and such rods having glossy butwavy surfaces. p I

The above and other objects, and the novel features of the'invention, will become apparent from the following description, havingreference to the accompanying drawings, wherein;

'2 Figs. 1, 2, and 3 are highly magnified schematic longitudinal sectional views of, respectivellil -a synthetic corundum rod in the as-grown condition, a similar rod partially glossed by, the process of the invention, and a fully glossed similar rod; Fig. 4 is a side elevational view, showing a heating torch suitable for performing the process of the invention;

Fig. 5 is a sectional view inFig. 4; I

Figs. 6 and 7 are reproductions of two Brush analysis charts showing, respectively, the

smoothness of a portion of the surface of a diamond-polished sapphire rod, and the smoothness of a sapphire rod glossed by the process of the invention; i

I Fig. 8 is an enlarged view of a syntheticcorundum rod having a small cavity with sharp-edges; Fig. 9- is an enlarged view of the. rodof mg; -,8 afterbeing glossed by the process of the invention; and I g g p v I Fig. 10 is an enlarged diagrammatic view illustrating the meaning of optic-orientation in a corundum rod. U v

Our process for glossing a body of corundum essentially comprises fusing a thin layer including the surface of the body ,While retainingits interior in the solid condition, and then resolidifying the fused layer. When a smoothand 7 even surface is desired, the rough surfacel'is heated rapidly until the projections, such as sharp crystal facets, melt and the molten. material coalesces and assumes an 'even and "continuous surface, after which heating is stopped and the surface material cools and resolidifies,

Unicrystalline rods of corundum vwhich :have been grown in rod form and exceed three sixteenth inch in diameter tend to be unstable and easily fractured. For best results," therefor,

corundum rods to be glossed shouldbe -threesixteenth inch or ;less in diameter. The surface of such a rod may be glossed over its full circumference, and along its entire lengthor any part-thereof, by holding itin a'flam'ean'dirotattaken along line 5-5 ing it on its longitudinal axis while eflecting relative translational movement between the flame shapes, such as slabs, also may be glossed in a gas flame.

A synthetic corundum rod B in the as-grown condition has a matte surface composed of many sharp projecting crystal facets, shown schematically at a in Fig. 1. When a partially glossed wavy surface free from scratches isdesired', shown schematically on a greatlyenlarged scale at b in Fig. 2, the operator removes each por-' tion of the rod from the name to discontinue heating thereof as soon as the crystal facets have been fused superficially and rounded over,

but before complete coalescence and the attain- I mentbf an even surfaceoccurs. The surface obtained in this way is glossy but not quite so shiny jand clear as a fully glossed surface. n p

Another and better way to form'a wavy sur-'- face is by heating successive portions of the surfaceof the body completely to fusion withfthe flame, while maintaining the flame source, sufficiently close to the surface thatthe dynamic gressively into regular waves. A fully glossed, shiny and. clear wavy surface results, 7 .When a glossy, clear, scratch-frea and substantially even surface is desired, ,as shown schematically on a greatly enlarged scale at c in Fig. 3, the rough surface of the rod is heated, in the flame until complete coalescence of the fused crystal facets occurs. By a substantially even surface is meant a surface which is practicallywithout elevation or depression, as comparedwith a rough unglossed surface and a partially glossed surface. However, perfect evenness cannot al- "ways beobtained because the original rough stone v "may have variations in its surface which are carried over to the glossed product.

Although it is permissible to use a single small flame for flame glossing corundum bodies, it is usually more desirable to use a multi-jetflame wherein the'jets intersect one another} in such a way as to give a resultant soft broad flame.

This is especially desirable for flame glossing a synthetic corundum rod by applying the flame progressivelyto successive zones along the length of the rod while it rotates on its longitudinal axis.

'A single flame jet sometimes produces a-barely discernible threaded appearance on' the surface of the rod under such'conditionsjbut'with a'soft broad flame formed by a plurality of 'intersect :ing flame jets no such thread'is obtained.

' 'As shown in Figs. 4'andi5l'one type of burner -Suitable-for flame glossing. is-forrned from a metal tube H bent into the shape of an arcor hook l3. One'end'off the'tube'is'sealed, as at perature flame which spreads transversely of the arc l3 and heats a substantial length of rod 18.

"Often it is desirable, prior to heat glossingthe '4 surface, to shape a corundum body by mechanical cutting or grinding with a conventional cutting or grinding wheel having diamond or other abrasive particles embedded therein, thereby making pits, scratches or other undesirable marks. For example, synthetickuni'cry's'talline corundum rods are usually ground to uniform diameter with abrasive diamond powder in a centerless grinder,

after which the ground slightly rough surfaces are heat glossed.

' some synthetic corundum rods include subsurface bubbles which require their classification as of inferior quality. When such inferior quality rod iscenterless ground to a uniform diameter, the bubbles near the surface are exposed as open cavities, as shown at 26 in Fig. 8. The

' I edges of such'cavities are so sharp that-they canaction of the gases in the flame'ripples the molten 'm-aterial, whichthen is allowed'to solidify pronot be used successfully where a smooth surface is required, as in guiding threads. However, when such a rod is flame glossed after centerless grinding, the sliarp'ed'ges ofi'the exposed bubbles become rounded and smooth, as shown at 21in Fig. 9, and the rest of the surface becomes-glossy and free from scratches.

Sometimes flame glossed corundum rods ,may become worn and cut after they have'b'eenin service for some time, for example 'asithreaicl guides. Such rough, worn and cut rods can be reglossed by applying a flame thereto in the manner described previously herein, anda smooth and glossy scratch-free surface comparable in quality to the original flame-glossed surface is again obtained.

The glossing of a corundum body ina flame is believed to proceed 'by' heating a thin layer, in-

cluding the surface, so rapidly that the layer fuses while the interior of the body is retained below the fusion temperature and remains solid" and rigid. Consequently, the original shape of such a body is retained without auxiliary support during flame glossing. It is thought'that'the force of surface tension acting on the fused material on Y the surface of the body l'evels'out irregularities, in spite of th counteracting force whicht'ends to make the materialirecrystall-ize into 'a definite angular shape, as would be expected from its crystal habit.

It has not been possible to measure accurately the temperatures of corundum surfaces during a flame glossing operation, but tests which have been conducted by exposing thin corundum rods to various carefully controlled temperaturesin a furnace have indicated that the surface must be heated to fusion if satisfactory glossingis to be obtained. For example, a sapphire rodwas heated at l900 C. in a furnace, but the rod after heating was only about 75% glossed, having-a satin finish rather than a completely glossy finish.

glossing synthetic corundum single crystals, it'is important to consider the optic orientationof the crystal. Corundum crystallizes; in the hexagonal system and has asingleprincipal optic or O-axis,

'represented'on the rod R inFig. l'0iby the line 0-0, which extends in the direction in which light may be 'passed through the-crystal without being doubly refracted. The optic orientation of a corundum boule or rod is defined by the angle 0 included between the C-axis C--C and the longitudinal or growth axis G-G, as shown in Fig. 10. When centerless ground synthetic clear sapphire rods, wherein the optic orientation 0- is between zero and '70 degrees, are selected for flame glossing, successful results are almost invariably obtained. However, when a centerless ground clear sapphire rod is selected'wherein the optic orientation is between 70 degrees and 90 degrees it is found that during the flame glossing operation a series of small notches forms along two sides of the rod. Therefore, it is important to select for flame glossing only those clear sapphire rods having an optic orientation between zero and 70 degrees. Synthetic ruby rods exhibit the notching phenomenon during flame glossing if the optic orientation exceeds 65 degrees. Therefore, the most inclusive range for flame glossing the various forms of synthetic corundum rods is zero to 65 degrees.

Synthetic unicrystalline corundum rods centerless ground and then heat glossed by the process of the invention had an average tensile strength of 102,000 lbs/sq. in., compared with 67,000 lbs/sq. in. for corundum rod in the asgrown condition. This superior strength of flame glossed rods is believed traceable to the elimination of a notch effect, which exists in the asgrown and diamond-polished rods.

Examination of heat-glossed synthetic corundum rods with an electron microscope at a magnification of 250021, has shown that the surfaces of such rods are free from scratches, pits, and striations. Rods which have been polished with fine diamond powder, on the other hand, show many scratches at 2500K magnification. Furthermore, the superior smoothness of flame polished corundum and sp-inel has been shown in Brush analysis tests with a Brush surface analyzer comprising a diamond point so mounted that when it rides on a surface the movement of the diamond point in a direction normal to the surface is transmitted through a piezo-electric crystal and an amplifier to a recording oscillograph. In the Brush analysis records shown in Figs. 6 and 7 magnification in the vertical direction is 40,000 and in the horizontal direction is 16. The displacement of the traced line 28 with respect to the datum line 29 in Fig. 6 definitely shows that surface irregularities occur at regular intervals on the surface of a centerless ground sapphire rod polished with diamond powder of between 2 and 4 microns size, whereas in Fig. 7 the traced line 3| coincides with the datum line 30, showing that the surface of a centerless ground and flame-glossed sapphire rod is perfectly smooth and even.

The glossy, clear, and scratch free surfaces on hard crystalline corundum bodies treated by the process of the invention make them useful for many purposes. For example, fully heat-glossed unicrystalline synthetic corundum bodies are useful as gemstones, jewel bearings, thread guides and tension posts, and as micrometer gauge contact points. Flame-glossed sapphire gauge contact points do not scratch work of a relatively soft nature, such as high lead content bearings, Whereas diamond polished points do tend to scratch such work because of the minute scratches formed on the surface by the particles of diamond. Flame-glossed sapphire thread guides outwear diamond polished sapphire guides. Partially glossed synthetic corundum rods, having a wavy surface, are advantageous as thread guides and tension posts because many threads can be run in parallel over such a guide without interfering with one another.

The process and products of the invention have been described specifically by way of illustration only. It is apparent, however, that variations in procedure and in the products may be made within the spirit and scope of the invention, as defined in the claims.

We claim:

1. A process for glossing a unicrystalline thin rod of corundum comprising selecting such a rod wherein the angle between the longitudinal and optic axes thereof is between zero and 65 degrees,

fusing a thin layer of such rod including the surface thereof by applying flame laterally against said rod and heating said surface therewith to a temperature of about 2050 C. while retaining the interior of said rod in a solid condition, and then resolidifying said layer to form a smooth glossy notchless surface.

2. A unicrystalline synthetic corundum rod grown in rod form and not exceeding three sixteenth inch in diameter, said rod having an optic orientation between zero and degrees and having a smooth, glossy, clear, notchless heat glossed surface over its full circumference and extending along at least the major portion of its length, said surface appearing to be completely scratch-free at a magnification of 2500X.

MALCOLM H. BARNES. EDWARD L. MoCANDLESS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,680,681 Herzog Aug. 14, 1928 1,852,840 Fuwa et al. Apr. 5, 1932 2,011,872 Rava Aug. 20, 1935 2,018,056 Delpech Oct. 22, 1935 2,371,486 Walker Mar. 13, 1945 2,405,892 Lederer et al Aug. 13, 1946 FOREIGN PATENTS Number Country A Date 243,251 Great Britain Nov. 26, 1925 

1. A PROCESS FOR GLOSSING A UNICRYSTALLINE THIN ROD OF CORUNDUM COMPRISING SELECTING SUCH A ROD WHEREIN THE ANGLE BETWEEN THE LONGITUDINAL AND OPTIC AXES THEREOF IS BETWEEN ZERO AND 65 DEGREES, FUSING A THIN LAYER OF SUCH ROD INCLUDING THE SURFACE THEREOF BY APPLYING FLAME LATERALLY AGANIST SAID ROD AND HEATING SAID SURFACE THEREWITH TO A TEMPERATURE OF ABOUT 2050* C. WHILE RETAINING THE INTERIOR OF SAID ROD IN A SOLID CONDITION, AND THEN RESOLIDIFYING SAID LAYER TO FORM A SMOOTH GLOSSY NOTCHLESS SURFACE.
 2. A UNICRYSTALLINE SYNTHETIC CORUNDUM ROD GROWN IN ROD FORM AND NOT EXCEEDING THREE SIXTEENTH INCH IN DIAMETER, SAID ROD HAVING AN OPTIC ORIENTATION BETWEEN ZERO AND 65 DEGREES AND HAVING A SMOOTH, GLOSSY, CLEAR, NOTCHLESS HEAT GLOSSED SURFACE OVER ITS FULL CIRCUMFERENCE AND EXTENDING ALONG AT LEAST THE MAJOR PORTION OF ITS LENGTH, SAID SURFACE APPEARING TO BE COMPLETELY SCRATCH-FREE AT A MAGNIFICATION OF 2500X. 